Variable resistors

ABSTRACT

Variable resistors having at least one fixed value resistance element, terminal caps, and one or more relatively rigid terminal lead rods provided with a contact element whereby the resistance can be varied by relative movement of the case and the terminal lead rod or rods. Internal ends of opposite terminal rods may be rigidly interconnected, either conductively or nonconductively relative to each other and/or the contact element. A plurality of the same or differing resistance elements may be electrically connected in series or parallel for maintaining the same and/or permitting variations in one or more electrical characteristics.

United States Patent [72] Inventor Charles S. Wright 5901 Fulton Ave.,Van Nuys, Calif. 91401 [21] Appl. No. 822,781 [22] Filed Mar. 20, 1969[45] Patented Nov. 16, 1971 Original application Mar. 18, 1968, Ser. No.714,108, now Patent No. 3,456,228, dated July 15, 1969, Continuation ofapplication Ser. No. 379,453, July 1, 1964, now abandoned. Divided andthis application Mar. 20, 1969, Ser. No. 822,78 1

[54] VARlABLE RESISTORS 4 Claims, 10 Drawing Figs.

[52] 11.8. C1 338/150, 338/176, 338/180 [51] 1nt.Cl 1'101c9/02 [50]Field of Search 338/92, 125,150,l52,176,177,178,179,180,181,182, 183,184, 197

[56] References Cited UNlTED STATES PATENTS 1,329,167 1/1920 Ebeling338/92 X Mountfordml 338/176X Primary Examiner-Laramie E. AskinAssistant ExaminerD. A. Tone Attorney-Allan M. Shapiro ABSTRACT:Variable resistors having at least one fixed value resistance element,terminal caps, and one or more relatively rigid terminal lead rodsprovided with a contact element whereby the resistance can be varied byrelative movement of the case and the terminal lead rod or rods.Internal ends of opposite terminal rods may be rigidly interconnected,either conductively or nonconductively relative to each other and/or thecontact element. A plurality of the same or differing resistanceelements may be electrically connected in series or parallel formaintaining the same and/or permitting variations in one or moreelectrical characteristics.

VARIABLE RESISTORS This a division of application Ser. No. 714,108,filed Mar. 18, 1968 now U.S. Pat. No. 3,456,228 issued July 15, 1969,which is a continuation of application Ser. No. 379,453, filed July 1,1964 and now abandoned.

The present invention relates to variable resistors and, moreparticularly, to resistors of a type having a fixed value resistanceelement and a selectively movable contact element for making electricalcontact with any desired spatially related points or regions along theeffective length of the resistance element whereby the effectiveelectrical resistance between terminals may be varied.

One of the objects of the present invention is the provision of avariable resistor whereby the effective electrical resistance may beselectively varied with effectively infinite resolution wherein thefixed resistance element and the electrical contact element arerelatively movable with respect to each other, either by maintaining thecontact element in a fixed position and moving the resistance elementwith respect thereto or, conversely, maintaining the resistance elementin a fixed position and moving the contact element with respect thereto,in either case such relative movement being either in the linear-slidingor spiral-rotating mode, so that variations in fixed resistance can beachieved by either the manufacturing consumer as he makes circuitadjustments or by the resistor manufacturer in supplying fixed valueresistors from resistor components having fixed value resistance elementwhich vary greatly in their manufactured values.

Another object of the present invention is the great reduction ofmanufacturing costs of precision resistors by the provision ofeconomical variable resistors which can utilize fixed value resistanceelements varying greatly in their manufactured resistance values. Forexample, the present state of the art of manufacturing precisionresistors, in quantity and other than using costly manufacturing,grading and selection techniques, provides resistors with tolerances inthe order of 0.01 percent from the nominal value, and the resistors mustbe manufactured to such individual precise nominal values; by means ofthe present invention, resistors may be manufactured in bulk with noprecision whatsoever and having nominal values in which the firstsignificant number varies by whole orders of magnitude (e.g., to 100ohms, 1 megohm to 10 megohms, and so forth), and the final effectiveresistance can be fixed at any desired nominal value therebetween withprecision accuracy in the order of 0.001 percent of such nominal value.

Another object of the present invention is the provision of a variableresistor in which the value of resistance is not temperature sensitive,i.e., having a temperature coefficient of resistance at leastapproaching zero.

Another object of the present invention is the provision of a variableresistor in which the effective temperature coefficient of resistancemay be selectively varied while maintaining a constant effectiveresistance.

According to the present invention, there is provided a variableresistor comprising nonconductive case means defining a longitudinalchamber substantially symmetrical about a longitudinal axis and open atits opposite ends, electrical resistance means secured to such casemeans and disposed within the chamber along substantially the entirelength thereof parallel to the longitudinal axis and having at least twoopposite ends adjacent respective case means ends, a plurality ofelectrical terminal means secured to respective case means ends, each ofthe terminal means being in electrical contact with at least one of theends of the electrical resistance means, each of the terminal meanscomprising a cap member in at least partial closure relationship to itscorresponding case means end, at least one of the cap members beingprovided with a central hole therethrough in alignment with thelongitudinal axis, and digitally movable contact means comprisinglongitudinal rod means partially disposed within the chamber along thelongitudinal axis and projecting outwardly therefrom through at leastsuch one central hole, and an electrically conductive contact elementmounted on the rod means in transverse symmetry and having a transverseperiphery at least a portion of which is in slidable engagement with theresistance means whereby selective manual longitudinal displacement ofthe movable contact means and the case means with respect to each othercauses corresponding variations in the effective electrical resistancecharacteristics presented between the plurality of electrical terminalmeans.

Specific features of various embodiments in accordance with the presentinvention include variations in the quantity, configuration and selectedcharacteristics of the resistance elements comprising the electricalresistance means, variations in the construction as well as thecross-sectional interior of the case means defining the chamber,variations in the configuration and mounting of the electrical contactelement, variations in the mechanical as well as electrical functionsand configurations of the electrical terminal means, and variations inthe electrical contact with and the configurations and mechanicalsupport of the longitudinal rod means.

The features of the present invention which are believed to be novel areset forth with particularity in the appended claims. The presentinvention, both as to its organization and manner of operation, togetherwith further objects and advantages thereof, may best be understood byreference to the following description, taken in connection with theaccompanying drawings, in which:

FIG. I is a side elevation view, partly broken away and longitudinallysectioned, of a variable resistor in accordance with the presentinvention;

FIG. 2 is a view similar in aspect to FIG. I of another embodiment ofthe present invention;

FIG. 3 is a similar view of a third embodiment of the present invention;

FIG. 4 is a transverse sectional view, partly in elevation, as seenalong line 4-4 in FIG. 3;

FIG. 5 is a longitudinally sectional side view, partly in elevation, ofa fourth embodiment of the present invention;

FIGS. 6, 7, and 8 are transverse sectional views of a cylindrical formof case member, showing some variations in the form of the resistanceelement;

FIG. 9 is a schematic diagram of a circuit attainable in accordance withthe present invention; and

FIG. 10 is a schematic diagram of another circuit attainable inaccordance with the present invention.

In the drawings, the same reference numerals occasionally refer to thesame or similar elements in the various embodiments. Further, it shouldbe understood that relative dimensions and proportions have beenexaggerated and otherwise distorted for convenience and clarity ofillustration and description.

Referring to FIG. 1, there is seen a variable resistor comprising anonconductive case means or member 10, preferably composed of a ceramicmaterial which defines a longitudinal chamber having a resistanceelement 12 disposed on the interior surface 14 of the case 10. Theconfiguration of the case 10, its interior surface 14 and the resistanceelement 12 may be varied, as will be described hereinafter in connectionwith later described embodiments; however, in FIG. 1, the case 10, itsinner surface 14 and the resistance element 12 are shown and describedas being tubular or cylindrical, FIG. 6 being an illustrative transversesectional view thereof. A longitudinal rod means is seen to comprise apair of axial terminal leads or rods 16 and 18 which are partiallydisposed within the chamber and project axially, i.e., along thelongitudinal center, out of the case 10 and are mechanically joinedtogether at their respective inner ends 20 and 22 by mechanical couplingmeans 24 comprising a collar member 26 composed of an electricallynonconductive material, such as ceramic, and a bonding material 28 suchas an epoxy cement. The terminal rod 16 is provided with an electricallyconductive contact element, indicated generally at 30, comprising anO-ring 32 having a conductive coat 34 thereon. The O-ring 32 ispreferably resilient and maybe composed of rubber or any similarmaterial, and the coat 34 is preferably composed of silver and/or gold,as by plating the O-ring 32 with a silver film and then plating with agold film, each film being approximately to 50 millionths of an inch inthickness. The transverse peripheral diameter of the contact element 30is sufficient to ensure continuous contact with the surface of theresistance element 12. The contact element 30 is mounted securely on theterminal lead 16 in any convenient manner such as, as illustrated, apair of heads 36 and 38 which bracket the contact element 30 and arecold formed in the terminal lead 16 itself, leaving a portion of theterminal lead end 20 projection therefrom for attachment of the couplingmeans 24 as aforesaid.

The electrical terminal means may comprise a pair of identicalelectrically conductive circular end caps 40 and 42 secured to theopposite ends of the case 10 as by press-fit engagement, each beingprovided with a boss portion 44 defining an axial or central hole 46 inslip-fit engagement with the terminal rods or leads l6 and 18 for bothmechanical support thereof and electrical contact therewith. In theembodiment illustrated in FIG. 1, each of the internal ends of the case10 is provided with an annular chamfered surface 48. The resistanceelement 12 may be formed in any convenient manner known in such art suchas, for example, vacuum deposition, carbon deposit or so forth, with theresistance element 12 overlapping from the inner surface 14 onto the endsurface 48 so as to form an adequate mechanical and electrical bond witha layer 50 of conductive material which is superimposed upon at leastthat portion of the resistance element 12 which overlaps the chamferedsurface 48. The conductive layer 50 is deposited, plated or otherwiseformed in any convenient manner for making a low-resistance contact withthe resistance layer 12 and providing a low-resistance contact for theend caps 40 and 42, the latter including an exterior lip portion 52which abuts against the outer end of the case 10 and has a cup-shapedintermediate portion 54 which joins the lip portion 52 and the boss 44to provide a press-fit mechanical and electrical engagement contact withthe layer 50 for secure mounting of the end caps as well as providing asolder contact region, indicated generally at 56, for the reception ofsolder in the manner and for the purpose as hereinafter describe inconnection with the operation of the device.

In the operation of the device illustrated in FIG. 1, axial orlongitudinal movement in either direction (as indicated by the arrow 58)of either the case 10 or the digitally movable contact means comprisingthe terminal leads l6 and 18 causes relative displacement of theresistance element 12 with respect to the electrical contact element 30.The original nominal or fixed magnitude of resistance of the resistor,as presented between the terminal and caps 40 and 42, is that of thefull resistance element 12 which resides between such end cups 40 and42; however, the effective electrical resistance of the resistor ispresented by that portion of the resistance element I2 residing betweenthe movable contact element 30 and the end cap 42 due to the shortcircuiting of the remainder of the resistance element 12 through theeffectively zero resistance path afforded by the contact meanscomprising its contact element 30 and its longitudinal terminal rod 16.Thus, relative axial or longitudinal displacement ofthe movable contactmeans with respect to the electrical resistance element 12 causesselective variation in the effective resistance presented by theresistor. One form of preferred mounting of the resistor is illustratedin FIG. 1 wherein the terminal rods 16 and 18 are immovably secured neartheir ends as by being bent to project through respective apertures 60and 62 in a circuit board 64 for mounting of the case 10 upon thesurface 66 of the circuit board 64. In this manner, the user of theresistor can merely mount the resistor in the position illustrated, makethe appropriate circuit connections on the circuit board to the terminalrods 16 and 18, and then adjust the value of the effective resistancerequired for proper circuit operation by merely moving the case 10 in anaxial direction 58in accordance with appropriate indications presentedby an convenient associated test equipment and/or the output of thecircuitry itself. If desired, and usually preferred, the user may. placea drop of solder in the region 56 in either or both of the end caps 40and 42 so as to solder the terminal leads to their respective end capsfor mechanical and electrical security once the desired effectiveresistance has been obtained. Of course, later readjustments can be madeby merely removing or loosening such solder connections by theapplication of the appropriate sol dering heat.

Another method of use relates to the fixed resistor field wherein themanufacturer supplies a fixed resistor having a nominal resistance valueand stated tolerances of accuracy of the nominal value, usually statedin plus and/or minus percentages of the nominal value. Of course, aprecisely accurate resistor requires smaller permissible tolerances,thereby requiring greater precision in the manufacture and/or selectionof resistance values with concomitant greater expense. By means of thepresent invention, resistors can be manufactured in a relatively inexactmanner insofar as the values of the resistance elements 12 areconcerned, with the nominal resistances varying greatly, and themanufacturer can make the adjustment of the effective resistance of theresistor to order within extremely low tolerance limits by merely makingthe aforesaid adjustment in the manner hereinabove described andsoldering either or both of the terminal rod leads l6 and 18 to arespective end cap 40 and/or 42 to provide a fixed value resistor. Themanufacturing adjustment can be relatively automated, as by theapplication of electrical balance bridge techniques. Still further,whereas the present state of the art of fixed value resistors is suchthat manufactured tolerances can be obtained in the order of 0.0lpercent of nominal value, the present invention permits the economicalattainment of more precise tolerances by at least a whole order ofmagnitude.

Referring to FIG. 2, there is seen an embodiment in accordance with thepresent invention comprising several modifications of the resistorillustrated in FIG. 1. The cylindrical inner surface 14 of the casemember 10 is provided with a resistance element 70 which has a spiralgroove 72 therethrough over a substantial portion but not all of itslength. The inner surface 14 of the case 10 is cylindrical throughoutits length, and the identical end caps 74 and 76 are washerlike in shapewith the boss 44 directed outwardly and the annular peripheries 78 and80 soldered to the annular conductive layers 48 for mechanicalengagement and electrical contact. The terminal lead 16 is threadedalong at least a portion of its length, as is the axial hole surface 82in the end cap 74, so that rotational movement of the case 10 and theterminal leads l6 and 18 with respect to each other causes spiralrelative movement of the contact element 30 with respect to theresistance element 70. Because of such spiral rotation, the rate ofaxial displacement is substantially less for any given amount ofrotational force applied than if such force were applied directly in anaxial direction, thus increasing the degree of digital control overaxial displacement for finer variations in adjustments of the effectiveresistance value of the resistor. Still further, the adjustments ineffective resistance value are made preferably over that portion of theresistance element 70 that is not spirally grooved so that even finervariations in resistance are attained.

Referring to FIG. 5, there is seen another embodiment of a resistor inaccordance with the present invention of the type more commonly known asa potentiometer, illustrated as mounted on a circuit board 62 and havingterminal means comprising a pair of terminal leads 84 and 86 in contactwith the resistance element 12 at opposite ends thereof by means ofrespective conductive rings 88 and 90 and extending through respectiveapertures 92 and 94 in the circuit board 62. The movable contact meanscomprises an electrical contact element 96 mounted on a longitudinallyor axially reciprocable terminal rod 98 by means of a pair ofwashers 100and 102 soldered at 104 and 106. The terminal rod 98 extends axially inboth directions out through the ends of the case 10 through a pair ofelectrically conductive mounting elements 108 and 110 which areelectrically insulated from the pair of conductive rings 88 and 9( bymeans of respective annular glass headers 112 and 114. The mountingmember 108 is provided with a terminal lead 116 which passes through anaperture 118 in the circuit board 62 in the same manner as the otherterminal leads 84 and 86.

The conductive rings 88 and 90 may be in any desirable shape orconfiguration for securing mechanical mounting of the terminal leads 84and 86 and electrical contact thereby with the resistance element 12. Inthe device illustrated, the axial mounting member 110 need not beelectrically conductive but is so shown as a construction convenience,the end configurations being generically analogous to the end capsdescribed in connection with the other embodiments. The

contact element 96 may be the same as the previously described contactelement 30 or may comprise a structure commonly known as a fuzz button,being gold-plated steel wool of find texture and highly compressed so asto form a substantially integral structure of cylindrical shape.

In operation, the electrical contact element 96 is caused to belongitudinally displaced with respect to the electrical resistanceelement 12 by axial or longitudinal displacing movement of the rod 98with respect to the case in the manner previously described. The rod 98functions as an electrical terminal because of its electrical connectionwith the terminal lead 116 so that the potential occuring at the latter,as referenced to the potential appearing at either of the otherelectrical terminals 84 or 86, represents the potential obtained by theselectively displaced location of the contact 96 of the resistanceelement 12.

Referring to FIG. 6, there is seen a transverse section of one form ofcase 10 which defines a cylindrical chamber and is provided with aresistance element 12 in the form of a layer of substantially uniformthickness and adhered or cohered onto the entire circumference of theinner surface 14 of the case 10, the layer 12 being such as has beendescribed as the resistance element 12, in FIGS. 1 and 5, and as theresistance element 70 in FIG. 2 prior to the formation of the spiralgroove 72. The present invention not being concerned with thecomposition of the resistance element except in the context of itsemployment in this invention, the resistance element may be formed inany convenient manner known in such art such as, for example, bytechniques commonly referred to as sputtering, vacuum deposition,dipping, and so forth, and may be composed of any appropriate metallicor nonmetallic material having electrical resistance properties orcharacteristics, either inherently and/or when disposed and/or treatedin accordance with the techniques of such art. For simplicity andconvenience of illustration, the previously described embodiments havebeen shown with a resistance element 12 or 70 of the general formillustrated in FIG. 6; however, it is expressly noted and should beunderstood that many other types, shapes and configurations ofresistance element or elements may be employed in any or all of suchembodiments with corresponding efficacy in accordance with theapplications; end uses and parameters of operation desired, all inaccordance with the present invention. Some additional configurationsand combinations will be described hereinafter.

Referring to FIG. 7, there is seen a cylindrical case 10 which isprovided with a longitudinal channel 120 in its inner surface 14 intowhich is bonded a resistance element 122 comprising a nonconductivesubstrate 124 having a resistance layer 126 formed or deposited thereonin any convenient manner known in the art, the planar shape being aconvenient form for many such techniques. The planar resistance element122 may be used in lieu of the cylindrical element 12 in either of theembodiments shown in FIGS. 1 and 5, without modification of the othermembers except to the extent that the hermetically sealed engagement ofthe end cups may be obtained by appropriate alteration of end capconfiguration, Thus, the present invention permits the usage ofresistance elements 122 which can be manufactured and suppliedseparately utilizing formation techniques not readily adaptable directlywithin the case 10.

Referring to FIG. 8, there is seen a modification of the combinationshown in FIG. 7 wherein the case 10 is provided with a pair of planarresistance elements 122 and 128 disposed in respective longitudinalchannels and 130 in the cylindrical inner surface 14, shown disposed indiametric opposition for security of electrical and mechanical contacttherewith but which may be disposed in any convenient circumferentiallyspaced relationship. Of course, an additional plurality of resistanceelements may be disposed in corresponding numbers of channels. It may benoted that the channels 120 and 130 provide a particularly convenientmeans of mounting the resistance elements 122 and 128.

Referring to FIGS. 3 and 4, there is seen another embodiment of aresistor in accordance with the present invention in which the case 132is rectanguloid in configuration and comprises an upper shell 134 and alower shell 136 which are joined together, as by epoxy cement 138 orother convenient material, to form a unitary case. The shells 134 and136 have respective planar longitudinal surfaces 140 and 142 opposingeach other and upon which opposing resistance elements 144 and 146 aredisposed. As shown, the longitudinal surfaces 140 and 142 may constitutesubstrate surfaces upon which the resistance elements 144 and 146 aredirectly and separately deposited or otherwise formed in accordance withthe techniques known in such art, or else the resistance elements 122and 128 of FIG. 8 may be disposed thereon,

A pair of end caps 74 and 76 are rectangular in end configuration andare soldered or brazed to respective conductive layers 48 for mechanicalengagement with the case 132 and electrical contact with respective endsof both of the resistance elements 144 and 146, as shown, so that suchresistance elements are in electrical parallel between the end caps 74and 76. The end cap 76 is provided with a terminal lead 148 extendingthrough a circuit board 150, and the end cap 74 is provided with a boss44 extendable through another circuit board 152 which is parallel to theopposing circuit board 150. The end cap 74 defines a central or axialhole 46 through which terminal rod lead 16 is reciprocable in slip-fitengagement. The terminal lead 16 terminates interiorly of the case 132and is provided at its interior end 154 with a contact element 156 ofC-shaped configuration. Preferably, the contact element 156 is slightlyresilient to ensure positive mechanical and electrical contact with bothof the resistance elements 144 and 146 in its slidable engagementtherewith.

In operation, the resistor illustrated in FIG. 3 may be adjusted byeither the manufacturer or the user for the desired effective resistancevalue by merely causing longitudinal displacement of the electricalcontact element 156 with respect to the resistance elements 144 and 146by selective longitudinal movement of either the terminal load rod 16 ofthe movable contact means or the case 132 with respect to the other,prior to insertion between the circuit boards and 152, and soldering thelead rod 16 to the boss 44 when the desired value is obtained, orinserting the resistor between such circuit boards and thenlongitudinally displacing the lead rod 16. Of course, appropriateelectrical terminal connection is made to the end cap 74 as at the boss44, which may be by printed circuit connection (not shown).

Referring to FIG. 9, there is seen a diagrammatic illustration of aresistor in accordance with the present invention which may have amechanical configuration in accordance with those embodiments previouslydescribed. A terminal means end cap 160 and its terminal lead 162 are inelectrical contact with one end of each of two resistance elements 164and 166 which may be disposed within the case in the manner illustratedin FIGS. 3, 4 and 8. The other terminal means end cap 168 and itsterminal lead 170 are in electrical contact with the opposite ends ofthe resistance elements 164 and 166 so that such elements are inparallel electrically between the terminal leads 162 and 170, Themovable contact element 172 and its terminal rod 174 are in electricalcontact with the end cap 168 and in slidable mechanical and electricalcontact with both of the resistance elements 164 and 166 along thelengths thereof. Thus, by longitudinal displacement movement of thecontact element 174 by means of its manual terminal rod 174 in either ofthe directions indicated by the arrow 58, the effective resistancepresented between the terminal leads 162 and 170 can be variablyadjusted. In accordance with a further aspect of the present inventionit is intended that, as another application of the inventive conceptsdescribed herein, the resistance elements 164 and 166 be chosen to haveequal nominal values of resistance but having equal and opposite valuesof temperature coefficient of resistance (hereinafter referred to aresistance t.c.") so that the effect of variation in temperature causedby changes in the ambient temperature and/or joulean heat will be inequal values but in opposite directions in the resistance elements 164and 166, thereby approaching a cancellation of such effects andresulting in a resistance device having a selectively variable effectiveresistance and a constant resistance t.c., a highly desirable andadvantageous combination of parameters. It being generally desirable tohave a resistor having as low a resistance t.c. as possible, for reasonswhich are well known in the art, considerable effort has been expendedin the past and is being expended today to perfect manufacturingtechniques so as to be able to consistently, predictably andeconomically produce resistance elements having negligible resistancet.c., one of the difiiculties being that the normal range of resistancet.c. obtained in a given group of manufactured elements variesconsiderably and unpredictably, with only a relatively small proportionof the elements having so called negligible resistance t.c.s. By meansof the present invention, no particular care or expense need beexercised in the manufacture or purchase of large groups of resistanceelements, it being merely a matter of selection of those elements havingequal but opposite resistance t.c.s for pairing in the resistor. Forexample, normal sorting techniques will permit the selection of aresistance element 164 having a resistance to of, say, +600 millionthand a resistance element 140 having a resistance t.c. of 600 millionthwhich, upon pairing in the manner shown in FIGS. 3 and 4 or 8, willcause the effective variable resistance to have an effective resistancetic. of0.36 a relatively negligible value.

Referring to FIG. 10, there is seen a diagrammatic illustration ofanother embodiment of a resistor in accordance with the presentinvention wherein the pair of resistance elements 164 and 166 again arechosen to have the same nominal vale of resistance and the same butopposing values of resistance t.c. such as previously described inconnection with FIG. 9. A terminal lead 176 is connected to one end ofonly one of the resistance elements 166, and another terminal lead 178is connected to the opposing end of only the other resistance element164. A movable contact element 180 is carried by its rod 182 in slidablemechanical and electrical contact with both of the resistance elements164 and 166 along the lengths thereof. The contact element 180 completesa series circuit path between the terminal leads 176 and 178 via thoseportions of resistance elements 164 and 166 between the point of contactof the contact element 180 and the respective terminal lead 178 and 176.Thus, by longitudinal displacement movement of the contact element 180by means of its manual rod 182 in either of the directions indicated bythe arrow 58, the amount of resistance added to or subtracted from thecircuit by means of resistance element 164 equals the amount ofresistance conversely subtracted from or added to the circuit by meansof resistance element 166, resulting in the effective resistancepresented between the terminal leads 178 and 176 being a constantnominal value. However, using the previous exemplary values of plus andminus 600 millionths as the resistance t.c. of resistance elements 164and 166, respectively, the selective longitudinal displacement movementof the contact element 180 causes the addition and subtraction ofopposing proportionate effective values of the resistance t.c.parameters of the resistance elements 164 and 166. Hence, the effectiveresistance t.c. of the constant value resistance presented betweenterminal leads 178 and 176 may be continuously varied and adjustedbetween the extremes of minus 600 millionths and plus 600 millionths bylongitudinal displacement location of the contact element 180 from theextreme left to the extreme right when viewed as in FIG. 10.Consequently, such a resistor has extremely valuable and novelapplications as a compensating element in circuitry wherein theoperating parameters are such that the other circuit elements require acounterbalancing resistance t.c. effect for proper and/or desiredfunction. Of course, the terminal rod 182 must be electrically insulatedfrom the terminal means leads 176 and 178, as by the glass header andcap arrangement shown in FIG. 5.

It should be understood that the variations in the features of thespecific embodiments illustrated are not meant to be restricted in theirutilization to only those combinations specifically illustrated. Inother words, features shown in connection with one embodiment aregenerally compatible with and applicable to the other embodimentsillustrated, in accordance within the teachings contained herein, andtheir interchangeability and application now will be obvious to anyonehaving ordinary skill in the art.

While particular embodiments of the present invention have been shownand described it will be obvious to those skilled in the art thatchanges and modifications ma be made without departing from thisinvention in its broader aspects, and, therefore, the aim in theappended claims is to cover all such changes and modification as fallwithin the true spirit and scope of this invention.

What is claimed is:

1. A variable resistor comprising:

nonconductive case means having a cylindrical interior surface defininga longitudinal chamber symmetrical about a longitudinal axis and open atit opposite ends;

electrical resistance means consisting of a cylindrical resistanceelement disposed within said chamber along substantially the entirelength thereof parallel to such longitudinal axis and bonded to saidinterior surface and having its to opposite ends adjacent respectivesuch case means ends;

a pair of electrical terminal means secured to respective such casemeans ends, each of said terminal means being in electrical andmechanical contact with a respective one of said opposite ends of saidresistance element, each of said terminal means comprising anelectrically conductive end cap member in closure relationship to itscorresponding case means end, each of said cap members being providedwith a central hole therethrough in alignment with such longitudinalaxis; and

digitally movable contact means comprising a pair of electricallyconductive longitudinal terminal rods partially disposed within saidchamber along such longitudinal axis and projecting outwardly therefromthrough respective ones of said central holes in longitudinalreciprocable relationship with respect to said case means and slip-fitmechanical engagement and electrical contact with respective ones ofsaid cap members, each of said terminal rods having an end portioninterior of said chamber, nonconductive coupling means mechanicallyjoining said terminal rods end portions, and an electrical contactelement concentrically mounted on one of said terminal rods adjacent itssaid end portion and having a circular periphery in slidable engagementwith said resistance element whereby selective manual longitudinaldisplacement of said movable contact means and said case means withrespect to each other causes corresponding variations in the effectiveelectrical resistance presented between said pair of electrical terminalmeans and said terminal rods, said terminal rods being adapted forelectrical circuit connections.

2. A variable resistor in accordance with claim 1 wherein:

said resistance element is of substantially uniform thickness and hasone end portion thereof provided with a spiral groove therethrough forcausing such portion to have greater resistivity than the remainder ofsaid resistance element whereby finer selectivity of variations in sucheffective electrical resistance is attained by longitudinal displacementof said electrical contact element along only the remainder portion ofsaid resistance element 3. A variable resistor in accordance with claim1 wherein:

each of said cap members is provided with a said central holetherethrough, one of such hole being internally threaded; and

one of said rods being in threaded engagement with such threaded capmember whereby said selective longitudinal displacement is by manualrotation of said movable con-

1. A variable resistor comprising: nonconductive case means having acylindrical interior surface defining a longitudinal chamber symmetricalabout a longitudinal axis and open at it opposite ends; electricalresistance means consisting of a cylindrical resistance element disposedwithin said chamber along substantially the entire length thereofparallel to such longitudinal axis and bonded to said interior surfaceand having its to opposite ends adjacent respective such case meansends; a pair of electrical terminal means secured to respective suchcase means ends, each of said terminal means being in electrical andmechanical contact with a respective one of said opposite ends of saidresistance element, each of said terminal means comprising anelectrically conductive end cap member in closure relationship to itscorresponding case means end, each of said cap members being providedwith a central hole therethrough in alignment with such longitudinalaxis; and digitally movable contact means comprising a pair ofelectrically conductive longitudinal terminal rods partially disposedwithin said chamber along such longitudinal axis and projectingoutwardly therefrom through respective ones of said central holes inlongitudinal reciprocable relationship with respect to said case meansand slip-fit mechanical engagement and electrical contact withrespective ones of said cap members, each of said terminal rods havingan end portion interior of said chamber, nonconductive coupling meansmechanically joining said terminal rods end portions, and an electricalcontact element concentrically mounted on one of said terminal rodsadjacent its said end portion and having a circular periphery inslidable engagement with said resistance element whereby selectivemanual longitudinal displacement of said movable contact means and saidcase means with respect to each other causes corresponding variations inthe effective electrical resistance presented between said pair ofelectrical terminal means and said terminal rods, said terminal rodsbeing adapted for electrical circuit connections.
 2. A variable resistorin accordance with claim 1 wherein: said resistance element is ofsubstantially uniform thickness and has one end portion thereof providedwith a spiral groove therethrough for causing such portion to havegreater resistivity than the remainder of said resistance elementwhereby finer selectivity of variations in such effective electricalresistance is attained by longitudinal displacement of said electricalcontact element along only the remainder portion of said resistanceelement
 3. A variable resistor in accordance with claim 1 wherein: eachof said cap members is provided with a said central hole therethrough,one of such hole being internally threaded; and one of said rods beingin threaded engagement with such threaded cap member whereby saidselective longitudinal displacement is by manual rotation of saidmovable contact means and said case means with respect to each other. 4.A variable resistor in accordance with claim 3 wherein: said resistanceelement is of substantially uniform thickness and has one end portionthereof provided with a spiral groove therethrough for causing suchportion to have greater resistivity than the remainder of saidresistance element whereby finer selectively of variations in sucheffective electrical resistance is attained by longitudinal displacementof said electrical contact element along only the remainder portion ofsaid resistance element.